Oxygen is a commodity chemical in the industrial gas industry. It has numerous applications including waste water treatment, glass melting furnaces, and the steel industry. One of the most common methods of oxygen production is by cryogenic distillation of air. However, this technology is not competitive for small size oxygen plants (&lt;100 TPD O.sub.2). The technology of choice for this size range is adsorption. There is a need in the marketplace to produce oxygen at low capital and energy costs by adsorptive gas separation.
Adsorptive processes are extensively used in the industry to produce oxygen from air for small size oxygen plants (&lt;100 TPD O.sub.2). There are two major categories of these processes--pressure swing adsorption processes (PSA) and vacuum swing adsorption processes (VSA). The pressure swing adsorption processes carry out the adsorption (feed) step at pressures much higher than ambient and adsorbent regeneration at pressures close to ambient. The adsorbent beds go through secondary process steps, such as pressure equalizations, depressurizations, blowdowns, and purge or various combinations of these during the cycle. Some of the O.sub.2 -PSA processes are described in U.S. Pat. Nos. 3,430,418; 3,636,679; 3,717,974; 3,738,087; 4,326,858; 4,329,158; 4,589,888; 4,650,501; 4,948,391; 4,969,935; 4,981,499; and U.K. Patent GB 2,227,685A.
These processes tend to be energy intensive and more suitable for smaller oxygen plants producing less than 40 tons of oxygen per day and preferably less than 20 tons of oxygen per day. A subset of O.sub.2 PSA processes is a rapid pressure swing adsorption (RPSA) process. As the name implies, this process involves similar steps as a PSA process, but carries out these steps very quickly. Some examples of this process are U.S. Pat. Nos. 4,194,892 and 4,406,675. Again, this process tends to be energy intensive and suitable for oxygen plants even smaller than O.sub.2 PSA's.
Primary reasons for high energy consumption in PSA processes are: (1) O.sub.2 recovery from these processes is low, and (2) the entire feed stream has to be compressed up to the adsorption pressure. These inefficiencies are somewhat circumvented in vacuum swing adsorption (VSA) processes. In these processes, adsorption is carried out at pressure close to ambient and adsorbent regeneration is carried out at sub-atmospheric levels. The adsorbent beds go through several secondary steps with the primary aim of increasing oxygen recovery and reducing adsorbent inventory per unit of product gas.
U.S. Pat. No. 3,957,463 describes an O.sub.2 VSA process comprised of the steps of: adsorption, evacuation and product repressurization. The process consists of two trains of two adsorbent beds in each train. The beds on the feed end of each train remove water and carbon dioxide from air, and the beds on the product end of each train remove nitrogen from air. Oxygen produced from the process is stored for later use as product and repressurization gas in a tank.
GB Patent 1,559,325 describes several two and three bed O.sub.2 VSA processes. The two bed O.sub.2 VSA processes have the steps: adsorption, evacuation, and product repressurization with the addition of purging the bed during evacuation and repressurizing it after evacuation with gas being continuously produced by the bed on adsorption step. The three bed options have similar steps with the addition that all the effluent gas from a bed toward the end of its adsorption step is fed to the bed which has finished product repressurization and is ready to go on to the air feed step. Effluent from the second bed is also withdrawn as oxygen product. A vacuum pump operates continuously in the three bed options, and the product take off is also continuous. GB Patent 1,594,454 describes the control strategy for O.sub.2 VSA process disclosed in GB Patent 1,559,325.
Japanese patent application 59-255060 (255,060/84) describes a four bed O.sub.2 VSA process with the process steps: adsorption, cocurrent depressurization, evacuation, vacuum purge, pressure equalization, and product repressurization. In this process, the gas obtained during the cocurrent depressurization step is used for the pressure equalization steps and then vacuum purge.
U.K. Patent application GB 2,154,895A describes three bed O.sub.2 VSA processes with process steps: adsorption, cocurrent depressurization, evacuation, vacuum purge, pressure equalization(s), and simultaneous feed repressurization with product end to product end pressure equalization. The cocurrent depressurized gas is used for pressure equalization(s) and vacuum purge.
Japanese patent application 1984-[Showa 59]-35,141 describes a three bed O.sub.2 VSA process with these steps: adsorption, evacuation with continuous purge, and repressurization. In this process, vacuum purge and repressurization are carried out by product oxygen.
U.K. Patent GB 2,109,266B describes three and four bed O.sub.2 VSA processes comprised of steps: adsorption, provide purge gas, evacuation, vacuum purge, and product repressurization steps. The purge gas used for vacuum purge step is provided either by cocurrent depressurization of the bed, which has finished its adsorption step, or by continuing the feed to the bed on its adsorption step but directing all the effluent from this bed to the bed on a vacuum purge step.
U.S. Pat. No. 3,986,849 describes a hydrogen PSA process using two to three pressure equalization steps prior to depressurization to provide purge in a process having multiple beds on overlapping adsorption steps.
U.S. Pat. No. 4,614,525 suggests an improvement to O.sub.2 VSA processes by heating the feed mixture by heat exchange with the vacuum pump.
U.S. Pat. No. 4,684,377 outlines a three bed O.sub.2 VSA process with steps: adsorption, simultaneous cocurrent depressurization and evacuation, evacuation, product end to product end pressure equalization by gas from the product end of the bed on simultaneous cocurrent depressurization, and evacuation step and product repressurization.
U.S. Pat. No. 4,756,723 describes an adsorptive process for oxygen production where adsorption is carried out at superambient pressure. This is followed by countercurrent depressurization, evacuation and product repressurization to adsorption pressure. Part of the gas discharged during the countercurrent depressurization step may also be used for pressure equalization with a bed before the product repressurization step.
U.S. Pat. No. 4,917,710 describes a two bed O.sub.2 VSA process with a product storage vessel. Process cycle steps are: adsorption, cocurrent depressurization, simultaneous cocurrent depressurization and evacuation, evacuation, vacuum purge by product, vacuum purge by gas obtained in a cocurrent depressurization step, simultaneous pressure equalization and product repressurization, and simultaneous feed and product repressurization. Gas for product repressurization and product purge is obtained from the product storage vessel. Gas for pressure equalization is obtained from the bed on simultaneous cocurrent depressurization and evacuation step.
U.S. Pat. No. 4,781,735 and European patent application 0 273 723 describe a three bed O.sub.2 VSA process with steps: adsorption, feed to feed or dual end pressure equalization, cocurrent depressurization, evacuation, vacuum purge by gas obtained in cocurrent depressurization step, product repressurization from bed on feed step, simultaneous feed repressurization and feed to feed or dual end pressure equalization.
European patent application 0 354 259 outlines various options for a two bed O.sub.2 VSA process: adsorption, cocurrent depressurization, evacuation, pressure equalization with gas obtained in cocurrent depressurization step and feed repressurization. Some options include vacuum purge by product gas from the bed on adsorption step.
U.S. Pat. No. 4,969,935 describes a three bed O.sub.2 VSA process with steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation, countercurrent evacuation, vacuum purge by product, product end to product end pressure equalization followed by product end to feed end pressure equalization using cocurrently depressurized gas and product repressurization.
U.S. Pat. No. 5,015,271 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation or feed, countercurrent evacuation, simultaneous product to product pressure equalization and feed repressurization, or vacuum purge, simultaneous feed and product repressurization and feed repressurization.
French Patent W091/12874; PCT/FR91/00164 describes a two bed O.sub.2 VSA process with basic process steps of adsorption, depressurization, evacuation, vacuum purge by product, pressure equalization and repressurization. Three variations are outlined.
European Patent 0 449 448 A1 outlines a two bed process with steps: adsorption, simultaneous evacuation and cocurrent depressurization, evacuation, product purge under vacuum, pressure equalization and product repressurization.
Despite the prior art, a need still exists for an O.sub.2 VSA process with higher oxygen recovery (i.e. lower energy costs) and lower adsorbent requirement per unit of oxygen production (i.e. lower capital costs) than the current processes. The present invention outlines a three bed vacuum swing adsorption (VSA) process to produce oxygen from air at higher oxygen recovery and lower adsorbent requirement per unit of oxygen product than current O.sub.2 VSA processes.